1. Field of the Invention
The present invention relates to heat recovery steam generators, and more particularly to a heat recovery boiler equipment provided with a low pressure evaporator and a high pressure evaporator.
2. Description of the Prior Art
The conventional heat recovery boiler of the type described comprises: for example, a high pressure boiler system including a high pressure super heater, a high pressure evaporator, a high pressure economizer and a high pressure boiler drum; a low pressure boiler system including a low pressure evaporator, a low pressure economizer and a low pressure boiler drum; and a water feed system for feeding water to the low pressure and the high pressure boiler drums, respectively, through the economizers in both the boiler systems. The above-mentioned high pressure super heater, high pressure evaporator and high pressure economizer as well as the low pressure evaporator and low pressure economizer are arranged in an exhaust gas flow path of a gas turbine in the described order for example, whereby waste heat contained in the exhaust gas is recovered to generate steam.
In the equipment as described above, at the time of start up of a gas turbine plant or a boiler, since the gas temperature is low for the high flow rate of the exhaust gas, heat recovery is mainly conducted in the low pressure evaporator and low pressure economizer, whereby, particularly, the temperature of feed water passing through the low pressure economizer is elevated and becomes higher than the saturation temperature of water corresponding to the internal pressure of the low pressure boiler drum to which the water is fed. As a result, water-steam mixture is formed, and when it is introduced into the drum, such a disadvantage is presented that steam is mixed into a down comer of the drum, thus normal circulation of the water is blocked resulting in burning damages to boiler tubes. Further, since water fed to the drum contains a considerable amount of steam, a product steam separated from the drum is mixed with mist, thus lowering the efficiency of separating water from steam in the drum. Further, in the case where evaporation occurs in the low pressure economizer, there occurs such a phenomenon that flowing in the water becomes so unstable as to give water hammering or the like in the economizer tubes, which is dangerous as it damages boiler equipment. In order to prevent water from steaming in the economizer, a pipe line is provided which circulates water from a water feed inlet of a low pressure drum or a high pressure drum to a condenser or a deaerater, thereby increasing the feed water flow rate to the economizer. However, with these methods, such a disadvantage is presented that the installation cost and operating cost are increased.